Centrifugal pump fuel system and method for gas turbine engine

ABSTRACT

A fuel system for a gas turbine engine that utilizes a centrifugal pump is presented. The system includes a fuel metering valve that is adapted to set a metered flow of fuel, and a throttle valve that is adapted to accurately control pressure drop across the fuel metering valve. The throttle valve has at least two variable orifices and a compensation chamber between the variable orifices. The throttle valve includes a differential valve piston slidable in a valve body. The differential valve piston comprises working surfaces of at least two different diameters such that changes in chamber pressures effect different axial forces upon the piston.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a divisional of copending U.S. patent Ser.No. 10/463,701, filed Jun. 16, 2003.

FIELD OF THE INVENTION

This invention pertains to gas turbine fuel systems and moreparticularly to gas turbine fuel systems that use high pressurecentrifugal pumps.

BACKGROUND OF THE INVENTION

There are two types of high pressure fuel pumping systems for gasturbine engines. The first type utilizes a positive displacement pump(typically a gear pump). The other type utilizes a centrifugal pump. Thefuel metering units for these types of fuel systems are substantiallydifferent in design, application and practice due to the fact thatpositive displacement pumps provide a predetermined flow rate based onpump speed (a flow generation source), whereas a centrifugal systemgenerates pressure (a pressure generation source) proportional to pumpspeed squared.

Examples of positive displacement pump fuel metering systems aredisclosed in U.S. Pat. No. 4,458,713 to Wernberg, U.S. Pat. No.5,433,237 to Kao et al., and U.S. Pat. No. 6,381,946 to Wernberg et al.In these systems, the speed of the pump determines the fuel flowsupplied to the fuel metering unit. For positive displacement systems,it is necessary for the fuel metering unit to recirculate (e.g. bypassand return) a portion of the pumped fuel flow back to the inlet of thehigh pressure pump. This is due to the fact that the pump is sized largeenough to provide enough fuel flow to meet the maximum demanded fuelflow rates for the gas turbine engine.

Centrifugal pumps, by contrast do not provide a predetermined flow ratebased upon speed. The fuel metering unit for centrifugal pumping systemsthrottles (restricts) pump flow rather than bypasses flow.

Referring to a prior art centrifugal system schematically shown in FIG.1, which generally depicts the relevant portions of a typicalcentrifugal pump type engine fuel system, the engine fuel systemincludes a fuel tank and a low pressure centrifugal boost pump. Theboost pump supplies fuel to a variable displacement starting pump and totwo high speed centrifugal pumps, one for the core engine and the otherfor the afterburner. The fuel for the high speed centrifugal pump forthe core engine is controlled with a fuel metering valve that ispositioned by an electrohydraulic servovalve (EHSV), which is in turncontrolled by the FADEC (full authority digital electronic controller).A position sensor (such as a LVDT or linear variable displacementtransducer) provides metering valve position feedback to the FADEC. Athrottle valve is arranged in series with the metering valve. Thethrottle valve provides a variable restriction orifice in the fuel flowpath that controls the pressure drop across the fuel metering valve (at50 PSI for example). The throttle valve opens and closes the variablerestriction orifice to maintain the pressure drop constant. To keep themetering valve pressure drop constant with excellent accuracy as istypically desired, the system of FIG. 1 employs a pressure sensor whichtypically contains a bellows or diaphragm that senses pressure dropacross the fuel metering valve. Typically, this pressure sensorpositions a low friction, low flow first stage valve which in turnpositions the larger throttle valve. This is mathematically anintegrating type system as flow from the first stage valve is integratedby the second stage throttle valve piston until the error in thepredetermined pressure drop is zero.

Unfortunately, incorporating the plumbing, multiple stages, valves andsensors to provide accurate control over metering valve pressure dropaccuracy such as that schematically illustrated in FIG. 1 has addedsubstantial weight, size, and expense. It has also reduced dynamicperformance, stability and the reliability of centrifugal pump meteringsystems. These are all disadvantages, particularly in aircraftapplications where there is always a constant desire to reduce weightwhile maintaining or increasing performance and reliability.

BRIEF SUMMARY OF THE INVENTION

It is a general objective of the present invention to provide animproved centrifugal pump metering system for gas turbine engine fuelmetering units utilizing high pressure centrifugal pumps.

In accordance with this general objective, one aspect of the presentinvention is directed toward an improved fuel system for a gas turbineengine that pumps fuel utilizing a high pressure centrifugal pump thatpumps fuel from a fuel supply. The system includes a fuel metering valvethat is adapted to set a metered flow of fuel, and a throttle valve thatis adapted to control pressure drop across the fuel metering valve. Thehigh pressure centrifugal pump, the fuel metering valve, and thethrottle valve are arranged in fluidic series with the nozzle outletpassage, which is adapted to convey fuel to the gas turbine engine fordischarge and combustion. The throttle valve has at least two variableorifices and a compensation chamber which senses the pressure betweenthe variable orifices. The throttle valve is movable to simultaneouslychange degrees of opening of the variable orifices. When the gas turbineengine and fuel system are operating, fuel pressure in the compensationchamber acts upon the throttle valve to control the position of thethrottle valve.

It is a further aspect of the present invention that the throttle valveincludes a multiple diameter valve piston slidable in a valve body. Thevalve piston comprises working surfaces of at least two differentdiameters such that changes in chamber pressures effect different axialforces upon the piston. The valve piston may comprise first and secondlands in spaced axial relation such that the throttle valve defines atleast three chambers, including a first chamber subjected to fluidpressure upstream of the fuel metering valve, a second chamber subjectedto fluid pressure downstream of the fuel metering valve, and thecompensation chamber intermediate of the two variable orifices. Thecompensation chamber is arranged in fluidic series with the fuelmetering valve and the nozzle outlet passage whereby fuel flows throughthe compensation chamber to the nozzle outlet passage.

It is an advantage that the compensation chamber may be used tocounteract variances in forces that can occur due to changes in valveposition. For example the valve experiences different amounts of fluidflow forces (namely, Bernoulli forces) and spring forces at differentvalve positions. Changes in fluid pressure in the compensation chambercan be designed to compensate for changes in spring forces and/ornaturally occurring fluid forces such as Bernoulli forces that may begenerated by fluid flowing through the throttle valve.

Another aspect of the present invention is directed toward a fuelmetering unit that can be used in a fuel system for regulating fuel flowin a gas turbine engine pressurized by a high pressure centrifugal pumpand delivered to a nozzle outlet passage. The fuel metering unitcomprises a fuel metering valve and a throttle valve arranged in fluidseries. The throttle valve comprises a valve body, a valve member and atleast two variable orifices. The valve member is movable in the valvebody to vary the size of the variable orifices. The variable orificesare arranged in fluidic series with a compensation chamber definedtherebetween such that a fluid control pressure develops in thecompensation chamber when fuel flows through the throttle valve. Fluidcontrol pressure which is developed in the compensation chamber actsupon the valve member to control position of the valve member.

Other aspects, objectives and advantages of the invention will becomemore apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the relevant portions of acentrifugal pump engine fuel system as may be found in a militaryafterburning fighter engine;

FIG. 2 is a schematic representation of a centrifugal pump fuel systemincorporating different fuel metering units for both the afterburner andcore sections of a gas turbine engine according to an embodiment of thepresent invention;

FIG. 3 is a cross sectional view shown partly in schematic form of thefuel metering unit schematically illustrated in FIG. 2 for the coresection of a gas turbine engine, with the throttle valve and meteringvalve in the closed position;

FIG. 4 is a similar view to that of FIG. 3, but with the throttle valveand metering valve in the open position;

FIG. 5 is an exploded isometric representation of a throttle valve usedin the fuel metering unit of FIS. 2-4; and

FIG. 6 is a cross sectional view shown partly in schematic form of thefuel metering unit schematically illustrated in FIG. 2 used for a vaporcore centrifugal pump and the afterburner of a gas turbine engine.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, two different fuel metering units 10, 12 accordingto two different embodiments of the present invention are illustratedfor a fuel delivery system 14 for a gas turbine engine 16 according toan embodiment of the present invention. One of the fuel metering units10 meters core engine fuel flow and is arranged to control primary fuelflow along a nozzle outlet passage 18 leading to nozzles that dischargeinto the core of the gas turbine engine 16. The other fuel metering unit12 meters afterburner fuel flow and is arranged to control fuel flowthrough a nozzle passage 20 leading to afterburner nozzles. Asillustrated, the main fuel metering unit 10 and the after burner unit 12are arranged in a parallel fluid circuit. It will be appreciated thatmany aircraft and gas turbine engines do not include afterburnersystems, and the invention is applicable and covers these fuel deliverysystems as well.

The fuel delivery system 14 includes a fuel supply 22 comprising a fueltank 24 and a low pressure centrifugal boost pump 26. The boost pump 26supplies fuel to each of the afterburner fueling system and the coreturbine fueling system. The booster pump 26 generates a low pressuresource Pb in a first conduit network section 27.

The core turbine fueling system includes a high pressure centrifugalpump 28 that pumps fuel toward the nozzle outlet passage 18. A startingpump system is provided to pump fuel flow at low engine speeds whenstarting the gas turbine engine. The starting pump system includes asmall variable displacement starting pump 30 in parallel circuit withthe centrifugal pump 28. The variable displacement start pump 30 pumpsthe fuel during initial engine startup when the engine speed is slow andthe high pressure centrifugal pump 28 is unable to generate sufficientpressure and/or flow. A control valve 32 is also provided to sensepressure or flow to control operation of the start pump 30, such thatthe start pump 30 may disengage or otherwise may stop pumping fuel upona predetermined pressure representing adequate engine speed. A checkvalve 34 arranged downstream of the high pressure centrifugal pump 28prevents fuel from backflowing through the centrifugal pump 28 at enginestart up when the speed is slow.

In either event, the centrifugal pump 28 and/or the start pump 30generate a high pressure source Ps/P1 in a second conduit section 36that leads toward the core of the gas turbine engine.

A fuel metering valve 40 is arranged in fluid series with thecentrifugal pump 28 to meter fuel flow through the nozzle outlet passage18. In this embodiment the fuel metering valve 40 is arranged downstreamof the high pressure centrifugal pump 28. The position of the fuelmetering valve 40 is set with a suitable servo-controller. For example,as shown in FIG. 1, an electro-hydraulic servo-valve (EHSV) 42, which isin turn controlled by the full authority digital electronic controller(FADEC) 44 as schematically indicated. Also, preferably, closed loopcontrol is provided over the fuel metering valve 40 with a positionsensor indicated as a linear variable displacement transducer (LVDT) 46providing electronic position feedback to the FADEC 44. The position ofthe fuel metering valve 40 sets the fuel flow rate flowing through thefuel metering valve 40 to the nozzle outlet passage 18. A small butsignificant pressure drop is also developed across the fuel meteringvalve 40 during operation (typically in a range of about 30-70 psi, butit could be significantly higher) which results in a reduced pressure P2in a third conduit section 48.

A throttle valve 50 is arranged in fluid series with the fuel meteringvalve 40 and the centrifugal pump 28 to regulate pressure drop acrossthe fuel metering valve 40. In this embodiment the throttle valve 50 isarranged downstream of the high pressure centrifugal pump 28 and thefuel metering valve 40. The throttle valve 50 includes a valve body 52and a movable valve member shown as a multiple diameter piston 54 (asshown in FIG. 3).

The valve body 52 may be comprised of an assembly of valve bodycomponents including an outer housing 52 a (which may include one ormore splits) and sleeve inserts 52 b, 52 c to provide for ready valveassembly. The valve body 52 defines a larger diameter bore 56 and asmaller diameter bore 58 to provide a valve body chamber 59 in which thepiston 56 slides. The valve body 52 defines an inlet port 60, an outletport 62, and a intermediate passage 64 through the valve body 52connecting inlet and outlet ports 60, 62. The piston 52 includes alarger cylindrical land 66 and a smaller cylindrical land 68, which maybe integrally connected by a shank portion 70 that provides for a fluidchamber therebetween. The combination of the piston 54 and the valvebody 52 define three separate chambers including an inlet chamber 69, acompensation chamber 71, and an actuation chamber 73.

The inlet chamber 69 is fluidically connected to the third conduitsection 48 and therefore receives metered fuel flow through the inletport 60. No restriction is provided at the inlet port 60 and thereforethe inlet chamber 69 is considered to be at pressure (P2). A spring 72in the inlet chamber 69 acts upon the piston 54 and biases the piston 54toward a closed position (as shown in FIG. 3) against a valve seat 74.The valve seat 74 may include an O-ring gasket 78 as shown to provide aseal and prevent leakage. Fluid pressure (P2) in the inlet chamber alsoacts upon the larger diameter land 66 also provides an axial force thaturges the differential piston 54 toward the closed position.

At the other axial end, fluid pressure in the actuation chamber 73 actsin opposition to the force of the spring 72 and the fluid pressure (P2)in the inlet chamber 69. The fluid pressure in the actuation chamber 73acts upon the smaller cylindrical land 68 to urge the differentialpiston 54 axially toward an open position (as shown in FIG. 4). Asolenoid valve 80 acts as a switch to connect the actuation chamber 73to a higher pressure conduit section 36 (at pressure P1) at the meteringvalve inlet or a lower sump pressure conduit section 27 (at pressurePb). The solenoid valve 80 includes an electrical coil 82 that drives aspool valve 84. The electrical coil 82 is electrically connected to theFADEC 44 for control thereby. In operation, the FADEC 44 selectivelysends signals to the solenoid valve 80 to pressurize the actuationchamber with pressure P1 or vent the actuation chamber to sump pressurePb.

The relative diameters of the lands 66, 68 of the differential piston 54are sized and the spring force sufficient such that the throttle valve50 closes when the actuation chamber 73 is vented to the lower sumppressure conduit section 27 (at pressure Pb). This is shown in FIG. 3where the solenoid valve 80 is positioned by the FADEC 44 to vent theactuation chamber to sump pressure Pb. In the closed position, the forceof the spring 72 (and/or fluid pressure at P1) seats the differentialpiston 54 against the seat 74 and the O-ring gasket 78 and o-ring gasket96 and thereby prevents fuel flow to the nozzle outlet passage 18. Thus,an advantage of the present invention is that the throttle valve 50 isbiased to a closed position and thereby may be used to provide automaticshut-off upon engine shut down or when otherwise desired. An additionallarge shut-off valve does not need to be provided in series with thethrottle valve 50, thereby providing for weight and size advantages.

In the embodiment of FIGS. 3-4, and during operation, the largercylindrical land 66 partially covers an intermediate port 87 disposedalong the intermediate passage 64 to control size of and provide for afirst variable restriction 88. As shown in FIG. 5, the intermediate port87 may comprise several holes 89 formed into the larger valve bodysleeve 52 b. The smaller cylindrical land 68 partially covers the outletport 62 to control size of and provide for a second variable restriction90. The outlet port 62 may comprise several holes 91 formed into thesmaller valve body sleeve 52 c.

The first and second variable restrictions 88, 90 are arranged in fluidseries in the throttle valve 50 between the nozzle outlet passage 18 (atpressure Pn) and the fuel metering valve. At selected positions of thethrottle valve 50 a pressure drop is developed across the firstrestriction 88. This generates a reduced pressure Pcomp (that is lessthan P1 and P2) inside the throttle valve in the compensation chamber71. This compensation pressure (Pcomp) is utilized to generate an axialcompensation force on the valve piston 54. In this embodiment, thecompensation pressure (Pcomp) is between the larger cylindrical land 66and the smaller cylindrical land 68. Since the larger cylindrical land66 has a larger working surface, more fluid pressure works upon thelarger cylindrical land 66 as opposed to the smaller cylindrical land68. The differential in working surface areas determines how much axialcompensation force is provided by the compensation pressure (Pcomp). Theforces on the valve can be represented by the following equation:F=P _(comp)*(IIR _(lg) ² −IIR _(sm) ²)+P _(actuation) *IIR _(sm) ² P ₂*IIR _(lg) ²−Spring Force±Fluid Force(s)where:

-   F=Axial Force on Valve Piston (which is zero when the valve is    balanced)-   P_(comp)=Compensation Pressure;-   P_(actuation)=Actuation Pressure (either P1 or Pb depending upon    state of solenoid valve);-   P₂=Pressure in Inlet Chamber;-   R_(lg)=Radius of larger diameter differential Piston Land;-   R_(sm)=Radius of smaller diameter differential Piston Land;

The second variable restriction 90 provides a pressure drop to reducethe pressure from Pcomp inside the compensation chamber 71 to Pn, thepressure in the nozzle outlet passage 18. Thus, two different pressuredrops occur across the throttle valve 50.

The present invention achieves a substantially constant pressure dropacross the metering valve 40. To achieve this, the ports 62, 87 thatrestrict flow and form restrictions 88, 90 in the throttle valve 50 areconfigured to control pressure Pcomp in the compensation chamber 71 andthereby generate a controlled axial compensation force upon the piston54. The shape, size and configuration of these ports 62, 87 (and morespecifically porting holes 89, 91 as shown in FIG. 5) are selected toprovide changes in compensation pressure that offset variances in springforces generated by the spring 72 that occur as the throttle valvepiston 54 moves axially and fluid flow forces that act upon the valve.Preferably the size of the restrictions 88, 90 (and thereby the variableporting orifices) change in flow area at different rates when thethrottle valve moves between positions. Generally for most operationalpositions, the upstream variable orifice has a larger flow area than thedownstream variable orifice during operation of the throttle valve(although there may be instances where this is not true). Spring forcechanges occur naturally since spring force is a function of position.The equation (Hooke's law) for determining changes in spring force is:ΔF=K*ΔX

-   -   where:    -   ΔF=the change in spring force;    -   K=the spring constant; and    -   ΔX=the change in spring/valve axial position        Likewise, naturally occurring fluid flow forces such as        Bernoulli forces can change based upon changes in valve        position. The present invention may be used to counteract        changes in fluid flow forces in addition to counteracting        changes in spring forces.

For engine starting, after sufficient pressure is available at the fuelcontrol, the FADEC 44 issues a signal that moves the shutoff solenoidvalve 80 to the run position. This connects the actuation chamber 73 tothe second conduit section and therefore pressure P1. At the same time,the FADEC 44 issues a signal to the EHSV 42 that holds fuel meteringvalve 40 in the closed position. This temporarily connects the inletchamber 69 to the sump pressure Pb through an annulus 92 formed into thefuel metering valve 40 that communicates with the supply conduit networksection 27 (at pressure Pb). The annulus 92 connects pressure Pb withthe inlet chamber 69 when the fuel metering valve 40 is in the closedposition (see e.g. FIG. 3). This causes a substantial pressure drop todevelop across the throttle valve 50 (for example P1-Pb may be about 250PSI). In turn, this causes the differential piston 54 of the throttlevalve 50 to move far to the right with the given orientation shown inthe Figures effectively closing the outlet port 62 with the smallerdiameter land 68 of the differential valve piston 54 (the throttle valve50 wants to lower the metering valve pressure differential bythrottling).

Shortly thereafter, the FADEC 44 then issues a signal to the EHSV 42 todrive the fuel metering valve 40 to a low metered flow position forengine starting. The opening of the fuel metering valve 40 connects theinlet chamber 69 to pressure P1 through the fuel metering valve 40(which is quickly reduced somewhat to pressure P2 by the pressure dropacross the fuel metering valve). Since the fluid pressure drop acrossthe throttle valve 50 is now near zero psi, the spring force of spring72 forces the differential piston 54 to the left (with the givenorientation of the Figures) and thereby opens the throttle valve 50.This allows pressure drop across the fuel metering valve 40 to increaseto a predetermined set point. For example, a pressure drop (P1-P2)across the fuel metering valve 40 of about 50 p.s.i. is typical for manyapplications.

The throttle valve 50 is now in regulation and is allowing metered fuelflow to flow therethrough to the nozzle outlet passage 18 and the coreof the gas turbine engine 16. In performing its regulating function, thethrottle valve will control the pressure drop (P1-P2) across the fuelmetering valve 40 and maintain it substantially constant at thepredetermined set point. In particular, if pressure P2 is too high ascompared with pressure P1, that excess pressure is communicated to inletchamber 69 tends to urge the throttle valve 50 further open which inturn relieves the pressure in the inlet chamber 69 and thereby reducespressure P2. Similarly, if pressure P1 is too high as compared with P2,the excess pressure is sensed or communicated to the actuation chamber73 which in turn restricts flow through the throttle valve 50 whichincreases pressure P2 to correct the variance in pressure drop from thepredetermined set point.

As the throttle valve 50 repositions itself to maintain a constantpressure drop across the fuel metering valve 40, the force of the spring72 changes due to axial movement. The pressure Pcomp generated in thecompensation chamber 71 is configured to offset those spring forcechanges. Pressure Pcomp is designed through configuration of thevariable restriction outlet port 62 and intermediate port 87 tocounteract changes in the spring force of spring 72 due to axial valverepositioning. As such changes in the compensation pressure Pcomp is afunction of change in axial position ΔX. As noted previously,compensation pressure Pcomp may also be designed to counteract the fluidflow forces that may be experienced that would otherwise tend to createsome error in addition to spring forces. The intent of the throttlingvalve is to maintain metering valve pressure drop (P1-P2) as constant aspossible for the entire engine fuel flow operating envelope. If theengine speed versus fuel flow requirements are known for both engineacceleration and deceleration conditions, the possible combinations ofinlet pressure P1, outlet pressure Pn, and P2 pressure can bemathematically determined by one of ordinary skill in the art.Substituting these values into the force balance and flow equations forthe valve the compensating pressure in chamber 71 and port area 87 canbe calculated to give nearly zero error in metering valve pressure drop(P1-P2).

For engine shutdown, the FADEC 44 issues a signal to move the shutoffsolenoid valve 80 to the off position in which the actuation chamber 73is vented to the supply conduit network section 27 at sump pressure Pb.At approximately the same time, the FADEC 44 issues a signal to the EHSV42 to drive the fuel metering valve 40 to the closed position exposingthe inlet chamber 69 to sump pressure Pb as well. Since fluid forces arenow generally balanced across the throttle valve 50, the spring 72 takesover and drives the differential valve piston 54 closed against valveseat 74. The gasket 78 at the valve seat 74 prevents leakage to theoutlet passage (as well as other strategically located seals 94, 96which may be needed depending upon how many components make up anassembly for the valve body).

It is an advantage in viewing the embodiment of the invention for thecore engine of FIGS. 2-5 that there is no need for a pressure sensor andintegrating valve which are arranged in parallel with the fuel meteringvalve (schematically shown in FIG. 1). The present invention achievesgood to excellent accuracy for maintaining a constant pressure dropacross the fuel metering valve while also reducing weight and expense,and at the same time increasing dynamic performance, stability andreliability of the fuel system Some or all of these advantages can beobtained with the present invention.

Another embodiment of the invention is shown in FIGS. 2 and 6,incorporated in an afterburner system for a gas turbine engine 16. Thesame general principles that apply to the first embodiment likewisegenerally apply to this embodiment. However, this embodimentdemonstrates that many design alterations and different valvearrangements can be made without departing from the present invention.

In this embodiment, a fuel metering valve 110 and a throttle valve 112are arranged in fluid series upstream of a high speed, high pressure,vapor core centrifugal pump 114. A check valve 116 and an overboarddrain valve 118 may be positioned downstream of the pump 114. Themetering and shutoff/throttling valves are located at the inlet of thevapor core centrifugal pump rather than the discharge of the pump so thepump can be drained of fuel when the afterburner is not being used. Thissaves energy and prevents high fuel temperatures in the non-flowingpump. The drained pump is kept rotating at high speed whenever theengine is in operation and it can be brought on-line very quickly byopening the metering valve and throttling/shutoff valve on the pumpinlet.

Like the first embodiment, the position of the fuel metering valve 110is set with an electro-hydraulic servo-valve (EHSV) 120, which is inturn controlled by the full authority digital electronic controller(FADEC) 44 as schematically indicated. Also like the first embodiment,closed loop control is preferably provided over the fuel metering valve110 with a position sensor indicated as a linear variable displacementtransducer (LVDT) 122 providing electronic position feedback to theFADEC 44. The position of the fuel metering valve 110 sets the fuel flowrate flowing through the fuel metering valve 110 to the nozzle outletpassage 20. A pressure drop is also developed across the fuel meteringvalve 40 during operation (typically in a range of about 30-70 psi butcould be much higher) which results a lower pressure Pbl in a secondconduit section 124 of the afterburner system.

The throttle valve 112 is arranged in fluid series with the fuelmetering valve 110 and the centrifugal pump 114 to regulate pressuredrop across the fuel metering valve 110. In this embodiment, thethrottle valve 112 is arranged downstream of the fuel metering valve110, but upstream of the high pressure centrifugal pump 114. With twodifferent arrangements being shown in different embodiments, it will beappreciated that the throttle valve, pump and fuel metering valve may bearranged in any number of different arrangements in fluid series withone another.

The throttle valve 112 includes a valve body 126 and a movabledifferential valve piston 128. Like the first embodiment, the valve body126 may be comprised of an assembly of valve body components. With thevalve piston 128 slidably mounted in the valve body 128, the combinationdefines three chambers 130, 132, 134 which may be in communication withor subjected to different pressures Pb1, Pb and Pcomp2 during operation.In this embodiment (like the first embodiment), a spring 136 is arrangedin the Pb1 chamber 130 to urge the valve piston 128 to open thethrottling port 148(Andy—The difference has to do with the shutofffunction. In the first case the throttling valve provides the shutofffunction and the spring first pushes the valve to open the throttlingport (62) and then continues to drive the piston to a stop where it thenprovides shutoff. In the second case the metering valve provides theshutoff function so the throttling valve does not require the shutoffseals.) Fuel shutoff in this embodiment is achieved with the fuelmetering valve 110. Specifically, the fuel metering valve 110 is movableagainst a valve seat 138 which may include an annular seal 140 forshutoff. Also the check valve 116 is arranged prevent fuel leakage tothe nozzles when fuel is shut off. There is also no need for a solenoidvalve or fluid switch in this embodiment.

Selected porting of the chambers in the throttle valve 112 is providedto control how the throttle valve regulates pressure. The spring chamber130 is connected by a port and passage 131 to the pressure Pb1 generatedbetween the throttle valve 112 and fuel metering valve 110. Thedifferential chamber 132 is connected by a port and passage 133 to thePressure Pb experienced upstream of the fuel metering valve 110 inconduit section 27. In this embodiment, the compensation chamber 134 isconnected by an inlet port 135 to the main fuel flow passage at pressurePb1 from the fuel metering valve 110 and an outlet port 137 connected tothe core inlet chamber 139 of the centrifugal pump 114. The vapor corecentrifugal pump 114 includes a rotor 142 that impels fuel from the coreinlet chamber 139 to a radial outlet 144 at pressure P1′, which is thencommunicated through nozzle outlet passage 20 to the afterburner nozzlesof the gas turbine engine 16.

As shown in FIG. 6, movement of the differential valve piston 128 opensand closes the inlet and outlet ports 135, 137 for the compensationchamber creating variable restrictions 146, 148 that control thecompensation pressure Pcomp2. The differential valve piston 128 includesthrough-ports 150, 152 that align with the inlet and outlet ports 135,137 to communicate fuel through the valve piston 128 into and out of thecompensation chamber 134. The inlet and outlet ports 135, 137 may beselectively configured in size and shape to control the size of thevariable restrictions 146, 148 over the stroke of the valve piston 128.Alternatively (or in addition) the through-ports 150, 152 of thedifferential valve piston 128 may be selectively configured to controlthe size of the variable restrictions 146, 148. In either event, andwhen the throttle valve 112 moves/repositions, the restrictions 144, 146cause the compensation pressure Pcomp2 to change in a manner thatcounteracts forces and/or fluid forces to maintain substantiallyconstant the pressure drop across the fuel metering valve 10.

In this embodiment, there is no need for a separate shutoff solenoid,since the metering valve provides the shutoff function. The throttlevalve 112 automatically moves to a pressure regulating position when themetering valve opens and sufficient pressure and fuel flow is availablefrom the boost pump.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

1. A fuel metering unit for a fuel system for regulating fuel flow in agas turbine engine pumped by a high pressure centrifugal pump anddelivered to a nozzle outlet passage, the fuel metering unit comprising:a fuel metering valve and a throttle valve arranged in fluid series, thethrottle valve having a valve body, a double diameter valve member andat least two variable orifices, the valve member being movable in thevalve body to vary the size of the variable orifices, the variableorifices arranged in fluidic series with a compensation chamber definedtherebetween such that a fluid control pressure develops in thecompensation chamber when fuel flows through the throttle valve, fluidcontrol pressure acting upon the valve member to control position of thevalve member.
 2. The fuel metering unit of claim 1, wherein the throttlevalve includes a differential valve piston slidable in a valve body, thedifferential valve piston comprising two different diameters to providedifferently sized working surfaces across the throttle valve.
 3. Thefuel metering unit of claim 2, wherein the differential valve pistonfurther comprising first and second lands in spaced relation such thatthe throttle valve defines at least three chambers, including a firstchamber subjected to fluid pressure upstream of the fuel metering valve,a second chamber subjected to fluid pressure downstream of the fuelmetering valve, and the compensation chamber intermediate of the twovariable orifices, the compensation chamber arranged in fluidic serieswith the fuel metering valve and an outlet passage, the outlet passageadapted to convey fuel toward the gas turbine engine.
 4. The fuel systemof claim 1, wherein the variable orifices change in flow area atdifferent rates when the throttle valve moves between a plurality ofpositions.
 5. The fuel system of claim 4, wherein the variable orificesare in series between the fuel metering valve to include an upstreamvariable orifice and a downstream variable orifice, the upstreamvariable orifice having a larger flow area than the downstream variableorifice during operation of the throttle valve.
 6. The fuel system ofclaim 1, wherein the throttle valve comprises a spring urging thethrottle valve to a closed position preventing fuel from flowing to thenozzle outlet passage, and wherein the fuel system is free of a separateshut off valve.
 7. The fuel system of claim 6, further comprising aelectrically operated valve fluidically adapted to receive a highpressure fuel output from the high pressure centrifugal pump duringoperation, the electrically operated valve having an on position adaptedto connect the high pressure fuel output to an actuation chamber in thethrottle valve and a off position adapted to disconnect the actuationchamber from the high pressure fuel output and connect the actuationchamber to a sump or boost pressure, the high pressure fuel output wheninitially communicated to the actuation chamber via the on position ofthe electrically operated valve acting upon the throttle valve againstthe spring to move the throttle valve from the closed position to anopen position, the sump or boost pressure when communicated to theactuation chamber via the off position of the electrically operatedvalve allowing the spring to move the throttle valve from the openposition to the closed position.
 8. The fuel system of claim 1, whereinthe throttle valve is arranged downstream of the fuel metering valve. 9.The fuel system of claim 1, wherein the throttle valve is biased by aspring, the spring providing a variable biasing force determined by theposition of the throttle valve, wherein the two variable orifices changein flow area when the throttle valve moves when in operation wherebyfluid pressure in the compensation chamber is changed in a predeterminedmanner determined by a selective sizing of ports for the variableorifices, the change in fluid pressure being correlated to changes inthe variable biasing force wherein the variable biasing force and thefluid pressure in the compensation chamber generally increase anddecrease in unison.
 10. The fuel system of claim 9, wherein changes influid pressure in the compensation chamber further compensates for fluidforces such as Bernoulli forces generated when fluid is flowing throughthe throttle valve.
 11. The fuel system of claim 1, further comprising aservo controller positioning the fuel metering valve.